Question 6A130

The screen grid in a tetrode serves two primary functions. Firstly, it is placed between the control grid and the plate and is typically held at a positive DC potential while being RF-grounded via a capacitor. This electrostatic shielding significantly reduces the interelectrode capacitance between the control grid and the plate (grid-to-plate capacitance, Cgp). Reducing Cgp minimizes unwanted feedback that can cause instability and oscillation in RF amplifiers, making neutralization unnecessary except at very high frequencies. Secondly, the screen grid also shields the electron stream from the influence of the plate voltage. This means that the plate current becomes substantially independent of the plate voltage over a wide operating range. With plate current primarily controlled by the control grid and screen grid potentials, small changes in control grid voltage can produce large changes in plate current with minimal influence from the plate, resulting in much higher voltage amplification compared to a triode.